Our Changing Climate

Observed Changes in Global Climate

Global climate is changing rapidly compared to the pace of natural variations in climate that have occurred throughout Earth’s history. Global average temperature has increased by about 1.8°F from 1901 to 2016, and observational evidence does not support any credible natural explanations for this amount of warming; instead, the evidence consistently points to human activities, especially emissions of greenhouse or heat-trapping gases, as the dominant cause.

Long-term temperature observations are among the most consistent and widespread evidence of a warming planet. Global annually averaged temperature measured over both land and oceans has increased by about 1.8°F (1.0°C) according to a linear trend from 1901 to 2016, and by 1.2°F (0.65°C) for the period 1986–2015 as compared to 1901–1960. The last few years have also seen record-breaking, climate-related weather extremes. For example, since the Third National Climate Assessment was published,1 2014 became the warmest year on record globally; 2015 surpassed 2014 by a wide margin; and 2016 surpassed 2015.2,3 Sixteen of the last 17 years have been the warmest ever recorded by human observations.

For short periods of time, from a few years to a decade or so, the increase in global temperature can be temporarily slowed or even reversed by natural variability (see Box 2.1). Over the past decade, such a slowdown led to numerous assertions that global warming had stopped. No temperature records, however, show that long-term global warming has ceased or even substantially slowed over the past decade.4,5,6,7,8,9 Instead, global annual average temperatures for the period since 1986 are likely much higher and appear to have risen at a more rapid rate than for any similar climatological (20–30 year) time period in at least the last 1,700 years.10,11

Box 2.1: Natural Variability

The conditions we experience in a given place at a given time are the result of both human and natural factors.

Long-term trends and future projections describe changes to the average state of the climate. The actual weather experienced is the result of combining long-term human-induced change with natural factors and the hard-to-predict variations of the weather in a given place, at a given time. Temperature, precipitation, and other day-to-day weather conditions are influenced by a range of factors, from fixed local conditions (such as topography and urban heat islands) to the cyclical and chaotic patterns of natural variability within the climate system, like El Niño. Over shorter timescales and smaller geographic regions, the influence of natural variability can be larger than the influence of human activity.10 Over longer timescales and larger geographic regions, however, the human influence can dominate. For example, during an El Niño year, winters across the southwestern United States are typically wetter than average, and global temperatures are higher than average. During a La Niña year, conditions across the southwestern United States are typically dry, and global temperatures tend to be cooler. Over climate timescales of multiple decades, however, global temperature continues to steadily increase.

How will global climate—and even more importantly, regional climate—change over the next few decades? The actual state of the climate depends on both natural variability and human-induced change. At the decadal scale, these two factors are equally strong.202 Scientific ability to predict the climate at the seasonal to decadal scale is limited both by the imperfect ability to specify the initial conditions of the state of the ocean (such as surface temperature and salinity) and the chaotic nature of the interconnected earth system.203,204 Over longer time scales (about 30 years, for global climate indicators; see Box 2.2), the human influence dominates.205 As human forcing exceeds the influence of natural variability for many aspects of Earth’s climate system, uncertainty in human choices and resulting emissions becomes increasingly important in determining the magnitude and patterns of future global warming. Natural variability will continue to be a factor, but most of the differences between present and future climates will be determined by choices that society makes today and over the next few decades that determine emissions of carbon dioxide and other heat-trapping gases, as well as any potential large-scale interventions as discussed in DeAngelo et al. (2017).27 The further out in time we look, the greater the influence of these human choices on the magnitude of future warming.

While thousands of studies conducted by researchers around the world have documented increases in temperature at Earth’s surface, as well as in the atmosphere and oceans, many other aspects of global climate are also changing12,13 (see also EPA 2016, Wuebbles et al. 201710,14). Studies have documented melting glaciers and ice sheets, shrinking snow cover and sea ice, rising sea levels, more frequent high temperature extremes and heavy precipitation events, and a host of other climate variables or “indicators” consistent with a warmer world (see Box 2.2). Observed trends have been confirmed by multiple independent research groups around the world.

Box 2.2: Indicators

Observed trends in a broad range of physical climate indicators show that Earth is warming.

There are many different types of physical observations, or “indicators,” that can be used to track how climate is changing (Ch. 1: Overview, see Figure 1.2). These indicators include changes in temperature and precipitation as well as observations of arctic sea ice, snow cover, alpine glaciers, growing season length, drought, wildfires, lake levels, and heavy precipitation. Some of these indicators, especially those derived from air temperature and precipitation observations, have nearly continuous data that extend back to the late 1800s in the United States (Blue Hill Meteorological Observatory)206 and the 1600s in Europe (Central England Temperature Record).207 These document century-scale changes in climate. Satellite-based indicators, on the other hand, extend back only to the late 1970s but provide an unparalleled and comprehensive record of the changes in Earth’s surface and atmosphere. Various chapters in CSSR discuss the different types of observations that capture the interconnected nature of the climate system.

Taken individually, each indicator simply shows changes that are occurring in that variable. Taken as a whole, however, in the context of scientific understanding of the climate system, the cumulative changes documented by each of these indicators paint a compelling and consistent picture of a warming world. For example, arctic sea ice has declined since the late 1970s, most glaciers have retreated, the frost-free season has lengthened, heavy precipitation events have increased in the United States and elsewhere in the world, and sea level has risen. Each of these indicators, and many more, are changing in ways that are consistent with a warming climate.

Many lines of evidence demonstrate that human activities, especially emissions of greenhouse gases from fossil fuel combustion, deforestation, and land-use change, are primarily responsible for the climate changes observed in the industrial era, especially over the last six decades. Observed warming over the period 1951–2010 was 1.2°F (0.65°C), and formal detection and attribution studies conclude that the likely range of the human contribution to the global average temperature increase over the period 1951–2010 is 1.1°F to 1.4°F (0.6°C to 0.8°C;15 see Knutson et al. 201716 for more on detection and attribution).

Human activities affect Earth’s climate by altering factors that control the amount of energy from the sun that enters and leaves the atmosphere. These factors, known as radiative forcings, include changes in greenhouse gases, small airborne soot and dust particles known as aerosols, and the reflectivity (or albedo) of Earth’s surface through land-use and land-cover changes (see Ch. 5: Land Changes).17,18 Increasing greenhouse gas levels in the atmosphere due to emissions from human activities are the largest of these radiative forcings. By absorbing the heat emitted by Earth and reradiating it equally in all directions, greenhouse gases increase the amount of heat retained inside the climate system, warming the planet. Aerosols produced by burning fossil fuels and by other human activities affect climate both directly, by scattering and absorbing sunlight, as well as indirectly, through their impact on cloud formation and cloud properties. Over the industrial era, the net effect of the combined direct and indirect effects of aerosols has been to cool the planet, partially offsetting greenhouse gas warming at the global scale.17,18

Over the last century, changes in solar output, volcanic emissions, and natural variability have only contributed marginally to the observed changes in climate (Figure 2.1).15,17 No natural cycles are found in the observational record that can explain the observed increases in the heat content of the atmosphere, the ocean, or the cryosphere since the industrial era.11,19,20,21 Greenhouse gas emissions from human activities are the only factors that can account for the observed warming over the last century; there are no credible alternative human or natural explanations supported by the observational evidence.10,22

Figure 2.1: Both human and natural factors influence Earth’s climate, but the long-term global warming trend observed over the past century can only be explained by the effect that human activities have had on the climate.

Sophisticated computer models of Earth’s climate system allow scientists to explore the effects of both natural and human factors. In this figure, the black line shows the observed annual average global surface temperature for 1880–2017 as a difference from the average value for 1880–1910. The other lines show the contributions from individual natural and human factors, all natural factors, all human factors, and the combined effects of both natural and human drivers. Details on these factors are provided below (panel references and colors refer to the static version of the figure, available via the “View static image” link above):

The top panel (a) shows the temperature changes simulated by a climate model when only natural factors (yellow line) are considered. The other lines show the individual contributions to the overall effect from observed changes in Earth’s orbit (brown line), the amount of incoming energy from the sun (purple line), and changes in emissions from volcanic eruptions (green line). Note that no long-term trend in globally averaged surface temperature over this time period would be expected from natural factors alone.10

The middle panel (b) shows the simulated changes in global temperature when considering only human influences (dark red line), including the contributions from emissions of greenhouse gases (purple line) and small particles (referred to as aerosols, brown line) as well as changes in ozone levels (orange line) and changes in land cover, including deforestation (green line). Changes in aerosols and land cover have had a net cooling effect in recent decades, while changes in near-surface ozone levels have had a small warming effect.18 These smaller effects are dominated by the large warming influence of greenhouse gases such as carbon dioxide and methane. Note that the net effect of human factors (dark red line) explains most of the long-term warming trend.

The bottom panel (c) shows the temperature change (orange line) simulated by a climate model when both human and natural influences are included. The result matches the observed temperature record closely, particularly since 1950, making the dominant role of human drivers plainly visible.

Researchers do not expect climate models to exactly reproduce the specific timing of actual weather events or short-term climate variations, but they do expect the models to capture how the whole climate system behaves over long periods of time. The simulated temperature lines represent the average values from a large number of simulation runs. The orange hatching represents uncertainty bands based on those simulations. For any given year, 95% of the simulations will lie inside the orange bands. Source: NASA GISS.